But why exactly is pleasure not a good in itself? How would you explain this in your own words?

Short answer: ordinary pleasure is intimately linked with pain and suffering, to such an extent that they are two sides of the same coin, although this truth can only be recognized clearly after you have done some living. Nothing wrong with taking pleasure in the activities that fulfil your bodily needs: food, drink, sex and so forth. But to believe that you are your body, and that pleasuring that body is the aim of your life, will ultimately bring you nothing but heartbreak, as your body ages and decays, your friends die off or disperse, your riches are spent or stolen, and so on.

The crucial thing to understand is the distinction between what you control and what you do not control. You are the king of your own inner domain; your desires, your emotions and opinions, your imaginings, everything that makes up your inner world, is potentially under your full control. If you go outside that kingdom, you are at best a foreigner with no rights, and at worst a slave. You do not control your body, your supply of food, the friends or partners that you have, the fate of your loved ones, and so on. The outer cosmos and its processes, including your personal fate, are in the hands of the Gods.

However, common sense dictates that you are powerless in your own kingdom, and that you must journey outside it to obtain happiness. So you are taught to neglect your thoughts, the way you view the world, the ethical quality of your actions, and instead concentrate on going out, striving and accumulating things - persons, goods, influence - which will bring you immediate pleasure and some small measure of influence over the lands outside your kingdom.

What does the Stoic sage do? He recognizes that the only possibility of unperturbable peace resides in his own kingdom. Rather than going outside his lands and competing with the others, he journeys in the opposite direction, to the heart of his kingdom; he finds what Hadot calls the "inner citadel", sits on his throne and takes up his sceptre. At that moment he comes once again into full authority over his own domain.